EP2554699A1

EP2554699A1 - Steel sheet with high tensile strength and superior ductility and method for producing same

Info

Publication number: EP2554699A1
Application number: EP11762477A
Authority: EP
Inventors: Takashi Kobayashi; Yoshimasa Funakawa; Tetsuya Mega
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2010-03-30
Filing date: 2011-02-25
Publication date: 2013-02-06
Anticipated expiration: 2031-02-25
Also published as: CN102822371A; KR20120113789A; EP2554699B1; WO2011122237A1; CN102822371B; JP5003785B2; KR101287331B1; EP2554699A4; JP2011208226A

Abstract

The invention provides a high strength steel sheet having excellent ductility which can avoid sudden breakage when being press-formed and has TS of 700 to 900 MPa, and a method for manufacturing such steel sheets. The high strength steel sheet having excellent ductility has a chemical composition represented by, in terms of % by mass, C: 0.5 to 1.5%, Si: not more than 0.1%, Mn: 10 to 25%, P: not more than 0.1%, S: not more than 0.05%, Al: not more that 0.1%, Ni: 3.0 to 8.0%, Mo: not more than 0.1%, N: not more than 0.01%, and Fe and inevitable impurities: balance, and has a microstructure formed of recrystallized austenite grains with an average grain diameter of 5 to 30 µm or further has another microstructure at an area ratio of not more than 1%.

Description

[Technical Field]

The present invention relates to a high strength steel sheet used in industrial fields such as transport machinery, for example, automobiles. In particular, the invention relates to a high strength steel sheet having excellent ductility with a tensile strength (TS) of 700 to 900 MPa, and a method for manufacturing the same.

[Background Art]

From the viewpoint of global warming suppression, the reduction of carbon dioxide emissions has recently become an urgent challenge and there has increasingly been a stronger need than ever for an improvement in automobile fuel efficiency. This issue has been actively addressed by increasing the tensile strength of steel sheets that are vehicle body materials in order to reduce the thickness of component parts, thus achieving the weight reduction of vehicle bodies. However, increasing the tensile strength of steel sheets inevitably results in a decrease in terms of press-formability. Thus, the development of steel sheets exhibiting a high tensile strength and good press-formability has been promoted. Various multiple phase steel sheets such as ferrite-martensite dual phase steel sheets and retained austenite steel sheets having transformation induced plasticity have been used for automobile parts so far and have shown an effect.
It has been recently decided that the carbon dioxide restrictions will be further tightened in the near future, which has led to higher targets for the weight reduction of car bodies. Even parts difficult to form which have been conventionally made of steel sheets with TS of 540 MPa or less are required to be reduced in thickness. Thus, there has been a strong need for high strength steel sheets which have TS of 700 to 900 MPa and exhibit comparable press-formability to conventional steel sheets.
Under circumstances described above, studies have been carried out in order to manufacture high strength steel sheets from austenitic high manganese steel that has been infrequently used as a sheet material. Austenitic high manganese steel, which has austenite as the main phase even at room temperature, has been conventionally used as nonmagnetic steel or low-temperature steel. Because marked work hardening and very high ductility are produced by the twinning induced plasticity of austenite, however, new types of high ductility and high strength steel sheets utilizing these effects have been proposed.
For example, patent document 1 discloses a process for manufacturing high strength hot rolled steel sheets for automobile parts having excellent formability, in which a steel slab that has a chemical composition represented by, in terms of % by weight, C: not more than 1.0%, Si: 0.01 to 2.50%, Mn: 10 to 30%, sol. Al: 0.001 to 0.10%, P: not more than 0.05%, S: not more than 0.05%, and iron and inevitable impurities: balance, is heated to 1100°C or above, thereafter continuously hot finish rolled in such a manner that the total reduction in rough rolling and finish rolling is not less than 90%, the finishing temperature is not less than 800°C and the final sheet thickness is 1.1 to 5.0 mm, subsequently cooled to not more than 650°C at a cooling rate of 10 to 100°C/s, and thereafter coiled.
Patent document 2 discloses high strength lightweight steel strips or steel sheets with excellent cold formability containing, in terms of % by mass, C: not more than 1.00%, Mn: 7.00 to 30.00%, Al: 1.00 to 10.00%, Si: above 2.50% to not more than 8.00%, Al + Si: above 3.50% to not more than 12.00% and B: above 0.00% to less than 0.01%, and optionally containing Ni: less than 8.00%, Cu: less than 3.00%, N: less than 0.60%, Nb: less than 0.30%, Ti: less than 0.30%, V: less then 0.30% and P: less than 0.01%.
Patent document 3 discloses Fe-C-Mn based austenitic hot rolled steel sheets which contain, in terms of % by weight, C: 0.5 to 0.7%, Mn: 17 to 24%, Si: not more than 3%, Al: not more than 0.050%, S: not more than 0.030%, P: not more than 0.08% and N: not more than 0.1%, optionally together with one or more elements selected from Cr: not more than 1%, Mo: not more than 0.40%, Ni: not more than 1%, Cu: not more than 5%, Ti: not more than 0.50%, Nb: not more than 0.50% and V: not more than 0.50%, the balance being represented by Fe and inevitable impurities, and in which the recrystallization fraction is greater than 75%, the surface fraction of precipitated carbides is less than 1.5% and the average austenite grain diameter is less than 18 µm, the steel sheets having TS of greater than 900 MPa and TS x El (El: break elongation) of greater than 45000 MPa·%. Patent document 3 also discloses Fe-C-Mn based austenitic cold rolled steel sheets of the same composition as above which have TS of greater than 950 MPa and TS x El of greater than 45000 MPa·%.
Patent document 4 discloses a method for manufacturing high manganese nonmagnetic steel having excellent local deformability, in which a steel ingot or a steel slab that contains C: 0.15 to 0.70 wt%, Si: 0.10 to 3.00 wt%, Mn: 12 to 30 wt%, Ti: 0.01 to 0.10 wt%, and Fe and inevitable impurities: balance, and has C and Mn contents satisfying 60 x C wt% + Mn wt% ≥ 36 wt% and a cleanliness of not more than 0.03% with respect to the content of nonmetallic inclusions is heated to 1050 to 1250°C and thereafter hot rolled at a finishing temperature of 900°C.

[Citation List]

[Patent Document]

[Patent document 1] Japanese Unexamined Patent Application Publication No. 4-259325
[Patent document 2] Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2004-521192
[Patent document 3] Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2006-528278
[Patent document 4] Japanese Unexamined Patent Application Publication No. 5-171273

[Summary of Invention]

[Technical Problem]

However, the austenitic high manganese steel sheets disclosed in patent documents 1 to 4 have a problem in that a so-called plasticity instability phenomenon in which a work hardening behavior is destabilized in a highly distorted region is apt to occur so that press-forming the steel sheet tends to result in sudden breakage without giving rise to necking.
It is an object of the present invention to provide a high strength steel sheet having excellent ductility which can avoid sudden breakage when being press-formed and has TS of 700 to 900 MPa, and a method for manufacturing such steel sheets.

[Solution to Problem]

The present inventors carried out studies in order to achieve the above object, thus finding the following.

i) In order to avoid sudden breakage during press forming, it is necessary that the local elongation (l-El) measured with respect to a 13B test piece specified in JIS Z2201 be not less than 5%.

ii) In order to achieve l-El of not less than 5%, it is effective to add Ni at not less than 3% by mass and to form a microstructure that is formed of recrystallized austenite grains having an average grain diameter of not less than 5 µm.

The present invention has been made on the basis of the above finding. The invention provides a high strength steel sheet having excellent ductility which has a chemical composition represented by, in terms of % by mass, C: 0.5 to 1.5%, Si: not more than 0.1%, Mn: 10 to 25%, P: not more than 0.1%, S: not more than 0.05%, Al: not more than 0.1%, Ni: 3.0 to 8.0%, Mo: not more than 0.1%, N: not more than 0.01%, and Fe and inevitable impurities: balance, and has a microstructure formed of recrystallized austenite grains with an average grain diameter of 5 to 30 µm or further has another microstructure at an area ratio of not more than 1%.
For example, the high strength steel sheet according to the present invention may be manufactured by reheating a steel slab having the above chemical composition to a heating temperature of 1100 to 1300°C, then hot rolling the steel slab to a hot rolled steel sheet at a finishing temperature of not less than 800°C, cooling the hot rolled steel sheet to at least 600°C in such a manner that the cooling rate is not less than 20°C/s in the temperature range of 800°C or less, and coiling the steel sheet at a coiling temperature of not more than 600°C.
After coiling, scales may be removed and the steel sheet may be thereafter annealed at an annealing temperature of 750 to 1050°C and cooled in such a manner that the cooling rate in the range of temperatures from the annealing temperature to at least 450°C is not less than 10°C/s. Alternatively, scales may be removed after coiling, and the steel may be thereafter cold rolled, annealed at an annealing temperature of 750 to 1050°C and cooled in such a manner that the cooling rate in the range of temperatures from the annealing temperature to at least 450°C is not less than 10°C/s.
That is, the high strength steel sheet according to the invention is a steel sheet that has been hot rolled (hereinafter, steel sheet as hot rolled), a steel sheet resulting from annealing of the steel sheet as hot rolled, or a steel sheet resulting from cold rolling and subsequent annealing of the steel sheet as hot rolled.

[Advantageous Effects of Invention]

The present invention allows for the manufacturing of high strength steel sheets having excellent ductility which can avoid sudden breakage during press forming and have TS of 700 to 900 MPa. The inventive high strength steel sheet has an excellent balance between strength and ductility and thus can be used for the fabrication of parts which are difficult to be formed. The high strength steel sheet of the invention is highly suited for the weight reduction of automobile bodies.

[Description of Embodiments]

The high strength steel sheet having excellent ductility according to the present invention, and a method for manufacturing such steel sheets will be described in detail hereinbelow. The percentages "%" indicating the contents of components mean "% by mass" unless otherwise mentioned.

1) Chemical composition

C: 0.5 to 1.5%

Carbon is an essential element for the stabilization of austenite phase and plays a major role in increasing the tensile strength of steel. If the C content is less than 0.5%, the stabilization of austenite phase is insufficient and excellent ductility cannot be obtained. On the other hand, a C content in excess of 1.5% causes the precipitation of carbides which lowers ductility. Thus, the C content is 0.5 to 1.5%, and preferably 0.5 to 1.0%.

Si: not more than 0.1%

Silicon is an element that can be added for the purpose of deoxidation of steel. However, adding an excessive amount of silicon which brings about a Si content in the steel exceeding 0.1% results in a saturation of the deoxidation effect as well as an increase in the amounts of internal defects and surface defects due to the increase of inclusion formed. Thus, the Si content is not more than 0.1%. In order to obtain the deoxidation effect sufficiently, the Si content is preferably 0.01 to 0.1%.

Mn: 10 to 25%

Similarly to carbon, manganese is an essential element for the stabilization of austenite phase. If the Mn content is less than 10%, the stabilization of austenite phase is insufficient and excellent ductility cannot be obtained. On the other hand, a Mn content in excess of 25% results in a decrease in hot processability of steel, thus deteriorating the productivity of steel sheets. Thus, the Mn content is 10 to 25%, and preferably 15 to 25%. Further, in order to achieve the improvement of ductility by the twinning induced plasticity effect stably, it is preferable to control the C content and the Mn content so as to satisfy Expression (1): $32 \leq 20 x [C] + [Mn] \leq 36$

wherein [C] and [Mn] are the C content and the Mn content, respectively.
As described above, carbon and manganese affect the stabilization of austenite phase. The present inventors have studied the relationship between the stabilization of austenite phase and material properties, in particular the TS x El balance. The present inventors have found that the TS x El balance becomes particularly excellent when the C and Mn contents are within the inventive ranges and when the C content and the Mn content satisfy Expression (1). It is assumed that when the value of 20 x [C] + [Mn] is less than the lower limit of Expression (1), namely, less than 32, the austenite phase becomes instable and is apt to be transformed into martensite, and when the value of 20 x [C] + [Mn] is above the upper limit of Expression (1), namely, above 36, the stacking fault energy becomes so large that the twinning induced plasticity hardly occurs.

P: not more than 0.1%

If the P content exceeds 0.1%, steel toughness is lowered. Thus, the P content is not more than 0.1%, and preferably not more than 0.05%.

S: not more than 0.05%

If the S content exceeds 0.05%, hot processability of steel is lowered. Thus, the S content is not more than 0.05%, preferably not more than 0.02%, and more preferably not more than 0.01%.

Al: not more than 0.1%

Aluminum is an element that can be added for the purpose of deoxidation of steel. However, adding an excessive amount of aluminum which brings about an Al content in the steel exceeding 0.1% results in a saturation of the deoxidation effect as well as an increase in the amounts of internal defects and surface defects due to the increase of inclusions formed. Thus, the Al content is not more than 0.1%. In order to obtain the deoxidation effect sufficiently, the Al content is preferably 0.01 to 0.1%.

Ni: 3.0 to 8.0%

Nickel is the most important element in the present invention. Nickel has an effect of increasing the stacking fault energy of steel so as to stably produce the twinning induced plasticity effect, thus increasing ductility. In particular, this element is effective for suppressing the occurrence of plasticity instability in a highly distorted region as well as for increasing l-El of austenitic high manganese steel sheets. In order to obtain these effects sufficiently, it is necessary that the Ni content be not less then 3.0%. Adding nickel to a Ni content in excess of 8.0% results in a saturation of these effects and an increase in production costs. Thus, the Ni content is 3.0 to 8.0%, and preferably 3.0 to 6.0%.

Mo: not more than 0.1%

Molybdenum retards the recrystallization of steel and produces an austenite grain refinement effect, thereby contributing to the increase in tensile strength of steel. In order to obtain these effects, the Mo content is preferably not less than 0.01%. If the Mo content exceeds 0.1%, however, the tensile strength TS becomes excessively high in excess of 900 MPa and the ductility is markedly deteriorated. Thus, the Mo content is not more than 0.1%, and preferably not more than 0.05%.

N: not more than 0.01%

If the N content exceeds 0.01%, steel ductility is lowered. Thus, the N content is not more than 0.01%, and preferably not more than 0.005%.
The balance is represented by Fe and inevitable impurities.

2) Microstructure

The high strength steel sheet of the invention has a microstructure formed of recrystallized austenite grains with an average grain diameter of 5 to 30 µm or further has another microstructure at an area ratio of not more that 1%. The microstructure needs to be an austenite single phase in order to achieve high ductility utilizing the twinning induced plasticity of the austenite phase. In order to make sure that even in a highly distorted region the austenite phase exhibits twinning induced plasticity stably, the austenite grains need to be recrystallized grains in which the internal strain energy has been sufficiently released. If the average grain diameter of the austenite grains is less than 5 µm, it becomes difficult for a deformation twin to be formed in a highly distorted region, resulting in the occurrence of plasticity instability phenomenon. In view of this, the average grain diameter of the recrystallized austenite grains in the inventive high strength steel sheet is not less than 5 µm, and preferably not less than 10 µm. If the average grain diameter is in excess of 30 µm, it becomes difficult to obtain desired TS. Thus, the average grain diameter of the recrystallized austenite grains is not more than 30 µm.
In austenitic high manganese steel sheets such as the inventive steel sheets, microstructures other than the recrystallized austenite grains such as iron carbides and martensite phases are sometimes formed depending on the cooling rate after hot rolling or the cooling rate after annealing. In order to obtain high tensile strength and excellent ductility stably, it is preferable that the formation of such other microstructures be suppressed minimally. However, the object of the invention is not deteriorated as long as the area ratio of such other microstructures relative to all the microstructures is about 1% or less. That is, the inventive high strength steel sheet has a microstructure formed of recrystallized austenite grains with an average grain diameter of 5 to 30 µm or further has another microstructure such as iron carbide or martensite phase at an area ratio of not more than 1%. In other words, the high strength steel sheet of the invention includes a microstructure in which recrystallized austenite grains have an average grain diameter of 5 to 30 µm and the recrystallized austenite grains have an area ratio of not less than 99% relative to all the microstructures in the steel sheet.
Here, the average grain diameter of the recrystallized austenite grains was determined by taking SEM images of a microstructure that was located at 1/4 of the sheet thickness in a cross section parallel to the rolling direction of the steel sheet at a 1000x to 5000x magnification for several fields of view, and analyzing the images while performing phase identification by EBSD analysis. Whether the grains were recrystallized grains was judged based on whether or not the aspect ratio of the crystal grain shape was less than 2, and the judgment was verified while considering the estimated amount of strain in the grains obtained by EBSD analysis.

3) Manufacturing conditions

Preferred conditions for manufacturing the inventive steel sheets will be described below. However, the methods for manufacturing the inventive high strength steel sheets are not limited to those methods described below.

Heating temperature for steel slab: 1100 to 1300°C

If the heating temperature for a steel slab exceeds 1300°C, hot processability is lowered and an increased amount of energy is incurred to perform heating. On the other hand, heating at a temperature less than 1100°C results in an increase in the load incurred during hot rolling. Thus, the heating temperature for a steel slab is 1100 to 1300°C, and preferably 1150 to 1250°C. In performing steel slab heating, a steel slab that has been cooled to room temperature may be reheated or a steel slab that is being cooled after casting and still has a high temperature may be heated supplementarily or kept at such a high temperature.

Finishing hot rolling temperature: not less than 800°C

If the finishing hot rolling temperature is less than 800°C, recrystallization and grain growth do not proceed sufficiently and grains tend to remain unrecrystallized in the resultant hot rolled steel sheet. Further, such a low finishing hot rolling temperature causes an increase in the rolling load in the case where the steel sheet is cold rolled afterward. Thus, the finishing hot rolling temperature is not less than 800°C, and preferably not less than 850°C. On the other hand, a finishing temperature exceeding 1050°C tends to cause the crystal grains to become excessively coarse, often resulting in decreases in strength and ductility. Thus, the finishing temperature is desirably not more than 1050°C. In order to ensure such a finishing temperature, the steel sheet that is being rolled may be supplementarily heated using a heating device such as an edge heater or a bar heater.

Cooling rate after hot rolling: not less than 20°C/s for temperatures in the range of 800°C or less

If cooling after hot rolling is performed at a cooling rate of less than 20°C/s for temperatures of 800°C or less, iron carbides are precipitated during cooling so as to decrease ductility. Thus, it is necessary that the hot rolled steel sheet be cooled to at least 600°C in such a manner that the steel sheet is cooled at a cooling rate of not less than 20°C/s in the temperature range of 800°C or less. If the rate of cooling after hot rolling is more than 100°C/s, recrystallization does not complete at times. Thus, the rate of cooling after hot rolling is preferably not more than 100°C/s.
In the case where the finishing temperature is above 800°C, natural cooling (air cooling) may be performed for 1 to 10 seconds in the temperature range not lass than 800°C in order to promote recrystallization. In such a case too, the steel sheet is cooled to at least 600°C in such a manner that the steel sheet is cooled at a cooling rate of not less than 20°C/s in the temperature range of 800°C or less.

Coiling temperature: not more than 600°C

If the coiling temperature is in excess of 600°C, iron carbides are formed during gradual cooling after coiling, thereby decreasing ductility. Thus, the coiling temperature is not more than 600°C, and preferably not more than 550°C.
The steel sheet as hot rolled that is manufactured as described above may be directly used as the inventive high strength steel sheet. After the steel sheet as hot rolled is descaled or after the steel sheet as hot rolled is descaled and thereafter cold rolled, the steel sheet may be further annealed under the following annealing conditions. The removal of scales may be carried out by a common procedure such as pickling.
Annealing conditions: annealing temperature: 750 to 1050°C, cooling rate in the range of temperatures from the annealing temperature to at least 450°C: not less than 10°C/s
The steel sheet as hot rolled may be annealed at an annealing temperature of 750 to 1050°C in order to promote grain growth. Annealing is more preferably carried out at an annealing temperature of 800 to 1000°C.
In the case where the steel sheet as hot rolled is cold rolled into a desired steel sheet thickness and such a steel sheet is annealed, it is necessary that the steel sheet be annealed at an annealing temperature of 750 to 1050°C in order to make sure that the microstructure in the steel sheet will be formed of recrystallized austenite grains with an average grain diameter of 5 to 30 µm. If the annealing temperature is less than 750°C, recrystallization does not complete and sufficient ductility cannot be obtained. On the other hand, an annealing temperature exceeding 1050°C may cause the crystal grains to become excessively coarse, often resulting in decreases in strength and ductility. More preferably, annealing is performed at an annealing temperature of 800 to 1000°C. The cold rolling reduction is not particularly limited as long as a desired sheet thickness is obtained. However, the cold rolling reduction is desirably about 50 to 70% from the viewpoint of production efficiency.
Irrespective of whether cold rolling is carried out or not, cooling in such a manner that the cooling rate in the range of temperatures from the annealing temperature to at least 450°C is less than 10°C/s results in the formation of iron carbides and a consequent decrease in ductility. Thus, it is necessary that the steel be cooled in such a manner that the cooling rate in the range of temperatures from the annealing temperature to at least 450°C is not less than 10°C/s.
The steel according to the invention may be produced by melting with a converter or an electric furnace. The molten steel is formed into a slab by ingot making followed by slabbing, or by continuous casting. It is preferable to perform any of various preliminary treatments, secondary refining, slab surface conditioning and others in accordance with need. From the viewpoint of productivity, annealing is preferably carried out with a continuous annealing facility. The steel sheet as hot rolled or the annealed steel sheet may be subjected to any of various plating treatments without deteriorating the advantageous effects of the invention. The steel sheet as hot rolled, the annealed steel sheet or the plated steel sheet may be temper rolled in order to correct the shape or control the surface roughness. Further, the inventive steel sheet may be subjected to any of various surface treatments such as painting and coating.

[EXAMPLES]

Steel slabs of steels A to K that had chemical compositions described in Table 1 were each hot rolled into a hot rolled steel sheet with a sheet thickness of 3 mm under hot rolling conditions described in Table 2. After scales were removed by pickling, some of the steel sheets were further annealed under annealing conditions described in Table 2, or were cold rolled at a cold rolling reduction described in Table 2 and thereafter annealed under annealing conditions described in Table 2. Thus, steel sheets as hot rolled, as well as hot rolled and annealed steel sheets and cold rolled and annealed steel sheets Nos. 1 to 20 were prepared.
The prepared steel sheets were analyzed in order to examine the microstructures by the aforementioned method, thus determining the phase configuration and the average grain diameter of recrystallized austenite grains. In Table 3, the phase configuration indicates recrystallized austenite grains and another type of microstructure that was observed at an area ratio exceeding 1%, and also indicates recrystallized austenite grains alone when the area ratio of other microstructures was 1% or less. Separately, a 13B test piece specified in JIS Z2201 was sampled along the rolling direction and was subjected to tensile test in accordance with the method described in JIS Z2241, thereby determining TS, El, 1-EL and TS x El. The steel sheet was evaluated to be a high strength steel sheet having excellent ductility when TS x El thereof was 60 GPa·% or more.
The results are described in Table 3. It is shown that all the steel sheets in INVENTIVE EXAMPLES have microstructures formed of recrystallized austenite grains with average grain diameters of not less than 5 µm, exhibited TS of 700 to 900 MPa, 1-El of not less than 5% and TS x El of not less than 60 GPa·%, and proved to be high strength steel sheets with excellent ductility capable of avoiding sudden breakage during press forming. Further, it is shown that the tensile property value TS x El was particularly excellent when the above Expression (1) is satisfied.

[Table 1]

(mass%)
Steel	C	Si	Mn	P	S	Al	Ni	Mo	N	20×[C]+[Mn]	Remarks
A	0.5	0.05	24	0.01	0.003	0.04	5.0	0.01	0.003	34	Within inventive range
B	0.6	0.03	22	0.02	0.008	0.03	6.0	0.02	0.004	34	Within inventive range
C	0.8	0.03	18	0.02	0.004	0.03	3.5	0.02	0.005	34	Within inventive range
D	0.8	0.03	15	0.02	0.004	0.03	3.5	0.02	0.005	31	Within inventive range
E	1.0	0.01	16	0.03	0.007	0.05	4.0	0.05	0.005	36	Within inventive range
F	1.2	0.01	12	0.01	0.007	0.05	7.5	0.04	0.005	36	Within inventive range
G	1.4	0.08	10	0.02	0.005	0.02	6.0	0.08	0.003	38	Within inventive range
H	0.4	0.03	16	0.02	0.010	0.03	5.0	0.02	0.002	24	Outside inventive range
I	0.8	0.03	8	0.02	0.006	0.03	5.0	0.03	0.005	24	Outside inventive range
J	0.8	0.03	20	0.02	0.009	0.03	1.0	0.03	0.005	36	Outside inventive range
K	0.8	0.03	20	0.02	0.009	0.03	3.0	0.20	0.005	36	Outside inventive range

[Table 2]

Steel sheet	Steel	Hot rolling conditions					Cold rolling reduction (%)	Annealing conditions		Remarks
Steel sheet	Steel	Heating temperature (°C)	Finishing temperature (°C)	Air cooling time (s)	Cooling rate 1* (°Cls)	Coiling temperature (°C)	Cold rolling reduction (%)	Annealing temperature (°C)	Cooling rate 2* (°C/s)	Remarks
1	A	1250	940	0	50	500	0	-	-	As hot rolled
2	A	1250	800	0	50	500	0	850	10	Hot rolled and annealed
3	B	1200	920	0	30	550	0	-	-	As hot rolled
4	B	1200	820	5	30	600	0	-	-	As hot rolled
5	B	1200	780	0	30	550	0	-	-	As hot rolled
6	B	1200	920	0	15	600	0	-	-	As hot rolled
7	B	1200	920	0	30	650	0	-	-	As hot rolled
8	C	1200	860	0	40	450	0	-	-	As hot rolled
9	C	1200	840	0	40	450	60	900	20	Cold rolled and annealed
10	C	1200	840	0	40	450	60	1080	20	Cold rolled and annealed
11	C	1200	840	0	40	450	60	720	20	Cold rolled and annealed
12	C	1200	840	0	40	450	60	900	5	Cold rolled and annealed
13	D	1200	860	0	40	450	0	-	-	As hot rolled
14	E	1150	880	0	30	600	0	-	-	As hot rolled
15	F	1150	880	0	35	500	0	-	-	As hot rolled
16	G	1150	880	0	35	500	0	-	-	As hot rolled
17	H	1200	940	0	50	500	0	-	-	As hot rolled
18	I	1200	920	0	40	500	0	-	-	As hot rolled
19	J	1200	920	0	40	500	0	-	-	As hot rolled
20	K	1200	920	0	40	500	0	-	-	As hot rolled

*Cooling rate 1: rate of cooling from 800°C or below to coiling temperature
*Cooling rate 2: rate of cooling from annealing temperature to 450°C

[Table 3]

Steel sheet	Microstructures		Tensile properties				Remarks
Steel sheet	Phase configuration*	Average grain diameter of recrystallized γ grains (µm)	TS (MPa)	EI (%)	I-EI (%)	TS×EI (GPa·%)	Remarks
1	Recrystallized γ	15	833	84.0	10.5	70.0	INV. EX.
2	Recrystallized γ	6	887	74.1	7.1	65.7	INV. EX.
3	Recrystallized γ	14	825	83.9	8.6	69.2	INV. EX.
4	Recrystallized γ	7	881	74.5	6.0	65.6	INV. EX.
5	Recrystallized γ + unrecrystallized γ	4	945	58.1	3.4	54.9	COMP. EX.
6	Recrystallized γ + iron carbide	15	817	72.7	5.7	59.4	COMP. EX.
7	Recrystallized γ + iron carbide	13	833	69.0	5.4	57.5	COMP. EX.
8	Recrystallized γ	5	899	73.7	7.1	66.3	INV. EX.
9	Recrystallized γ	12	843	74.2	7.0	62.6	INV. EX.
10	Recrystallized γ	31	691	90.5	10.9	62.5	COMP. EX.
11	Recrystallized γ + unrecrystallized γ	3	962	52.5	2.8	50.5	COMP. EX.
12	Recrystallized γ + iron carbide	13	835	64.3	5.1	53.7	COMP. EX.
13	Recrystallized γ	7	877	68.7	6.6	60.2	INV. EX.
14	Recrystallized γ	12	887	77.3	7.9	68.6	INV. EX.
15	Recrystallized γ	14	776	89.5	10.3	69.5	INV. EX.
16	Recrystallized γ	13	824	73.8	7.6	60.8	INV. EX.
17	Recrystallized γ + M	11	965	49.7	2.7	48.0	COMP. EX.
18	Recrystallized γ + M	9	949	50.5	2.8	47.9	COMP. EX.
19	Recrystallized γ	12	951	73.6	3.5	70.0	COMP. EX.
20	Recrystallized γ	10	920	76.0	3.7	69.9	COMP. EX.

*γ: austenite, M: martensite

Claims

A high strength steel sheet having excellent ductility which has a chemical composition represented by, in terms of % by mass, C: 0.5 to 1.5%, Si: not more than 0.1%, Mn: 10 to 25%, P: not more than 0.1%, S: not more than 0.05%, Al: not more than 0.1%, Ni: 3.0 to 8.0%, Mo: not more than 0.1%, N: not more than 0.01%, and Fe and inevitable impurities: balance, and has a microstructure formed of recrystallized austenite grains with an average grain diameter of 5 to 30 µm or further has another microstructure at an area ratio of not more than 1%.
A method for manufacturing a high strength steel sheet having excellent ductility, comprising reheating a steel slab having the chemical composition according to Claim 1 to a heating temperature of 1100 to 1300°C, hot rolling the steel slab to a hot rolled steel sheet at a finishing temperature of not less than 800°C, cooling the hot rolled steel sheet to at least 600°C in such a manner that the cooling rate is not less than 20°C/s in the temperature range of 800°C or less, and coiling the steel sheet at a coiling temperature of not more than 600°C.
The method for manufacturing a high strength steel sheet having excellent ductility according to Claim 2, further comprising removing scales after coiling, annealing the steel at an annealing temperature of 750 to 1050°C, and cooling the steel in such a manner that the cooling rate in the range of temperatures from the annealing temperature to at least 450°C is not less than 10°C/s.
The method for manufacturing a high strength steel sheet having excellent ductility according to Claim 2, further comprising removing scales after coiling, cold rolling the steel, annealing the cold rolled steel at an annealing temperature of 750 to 1050°C, and cooling the steel in such a manner that the cooling rate in the range of temperatures from the annealing temperature to at least 450°C is not less than 10°C/s.